Several types of electronic systems require high voltage, high current pulses of electrical energy in order to operate. Usually, the pulses must have a very fast rise time and a high frequency rate of repetition. Typically, the pulses have a rise time of approximately 400 nanoseconds and a frequency of several kilohertz. To develop these output power pulses, a high power switch is required. In these types of electronic systems, the switch must be able to accommodate an output power pulse of several thousand volts and a couple hundred amperes.
Presently, such switches for these electronic systems are constructed using a thyratron tube. The thyratron tube acts as a switch between high power source and an output device. The rapid switching of the thyratron tube modulates the high power source to apply pulses of output power to the output device. The output pulse is usually inductively coupled to a load through the secondary winding of a transformer. In these systems, the switch and transformer are usually referred to as a "modulator."
In a particular type of electronic system, the load is typically a high power travelling wave tube. The output power pulses are applied to the high powered tube. The tube, as is well known, amplifies a low level microwave signal. The amplified high power microwave signal may then be radiated by an antenna.
The basic principle of operation of these modulators depends upon the storage of electrical energy in the primary winding of the transformer, and the subsequent discharge of a fraction or all of the stored energy through the secondary winding into the high powered vacuum tube. The type of modulator which discharges all of the stored energy during each pulse is referred to as a line type modulator which use pulse forming networks. Modulators which discharge only a small fraction of the stored energy are termed hard tube modulators, this term being taken from the use of the thyratron tube as the switch.
The main disadvantage of the line type modulator is that they operate at fixed pulse widths, and are limited in duty cycle and pulse repetition frequency. Additionally, their overall reliability is low due to the use of the thyratron, which exhibits a relatively short life.
Hard tube modulators are considerably more versatile with regards to pulse widths, duty cycle and pulse repetition frequency. However, their overall reliability is even worse than line type modulators due to the higher current, hence the higher stress, that the thyratron encounters.
The stress on the thyratron in the hard tube modulator is from the storage of electrical energy within the primary winding of the transformer. The primary winding is coupled in series with the switch and the power source. The switching of current in the primary winding develops considerable voltage and current stresses on the thyratron tube.
In the hard tube modulator, it would be highly desirable to eliminate the use of the thyratron and replace it with solid state switch which would take advantage of the high reliability and long life of solid state electronics as compared to thyratron tubes. However, a solid state device cannot by itself handle the current and voltage requirements of the hard tube modulator switch.